Healthy Indoors Magazine
Issue link: https://hi.healthyindoors.com/i/1355611
18 | March 2021 air. So, a lot of mixing means that it could take, in this case, about three times longer to change out most of the air. But what if you get little to no mixing? You'd get most of the old air removed in about 10 minutes at an air change rate of 6 ACH. How could that happen? By putting the air in slowly and having the entry and exit points well separated in the room. The ASHRAE Handbook of Fundamentals has a great chapter all about ventilation and infil- tration, and they call this type of air move- ment displacement flow. (See diagram below, from chapter 16 of the ASHRAE Handbook of Fundamentals.) At the other end of the mixing spectrum is entrainment flow. The diagram top of next page, also from the ASHRAE Handbook of Fundamentals, shows what that looks like. If you're in the HVAC industry, the word "entrainment" may be familiar to you. AC- CA's Manual T on air distribution uses that word in discussing the mixing of air in a room. The old air in the room is induced, or entrained, to move with the new (primary) air. The result is that the new and old air get What Poppendieck said is important, but he really just scratched the surface of this topic. His tweet referred to a room, but what about larger areas, like a zone or a whole building? What kind of air are we talking about here? Air being recirculated through the heating and cooling system? Air infil- trating and exfiltrating through the building enclosure? Ventilating with outdoor air? And what do we need to know about mixing? Mixing it up Poppendieck's point is that if you have a room full of air that has a certain concen- tration of pollutants, you probably don't re- move an equal amount of old air just by add- ing new air. Thus, you don't remove as many pollutants as you might think. In his tweets, Pop- pendieck takes a kind of worst-case scenario, one which you get a lot of mixing and remove less of the old air pollutants. He gave as an example the intro- duction of new air at the rate of 6 air chang- es per hour (ACH). The other way to look at air changes is to flip it from air changes per hour to hours per air change. So, 6 air changes per hour means that it takes 1/6 hours per air change. At that rate, he said, instead of changing all the air in the room in 1/6 of an hour (10 minutes), it could take "up to 0.5 hours to remove 95%" of the old L et's talk about one of my favorite subjects: air. Because of the glob- al COVID-19 pandemic and the novel coronavirus (SARS-COV-2) that spreads it, the indoor air quality (IAQ) community—researchers, mediators, man- ufacturers, bloggers, groupies—is having a moment. This must be the first time in his- tory that IAQ researchers have logged so many hours on television and been quoted so frequently in news articles. And they're tweeting up a storm, too. And that brings me to the tweet by IAQ researcher Dustin Poppendieck that served as the impetus for this article: That tweet was one of four in a short thread he put out earlier this week. His point was that the calculated air change rate is based on how much "new" air you put into a room, but that doesn't mean you're removing an equal amount of "old" air from the house. The difference is mixing. Some of the air removed will be the air you just put into the room, with the proportion of new- to-old air dependent on the amount of mix- ing that happens. And that got me thinking... Air Change Rates and IAQ by Allison Bailes III, Ph.D.